The present invention generally relates to an automatic transmission for automobiles, and, more particularly, to a breathing system therefor.
An automatic transmission for automobiles generally comprises a fluid torque converter, an oil pump, and an auxiliary gear means performing as a speed change gear mechanism, these structural elements having individual own housings which are air-tightly connected with each other thereby defining a closed space therein. This closed space is filled with air as well as oil serving as the working fluid of the fluid torque converter and as lubricant for the gear mechanism of the auxiliary gear means and other mechanical moving elements. The pressure in said closed space changes to a great extent in accordance with the temperature of the fluid existing in the space, dynamic pressure generated by the rotation of rotary elements contained in the assembly of the housings, etc. Therefore, such a housing assembly requires a breathing system for releasing said closed space to the atmosphere. In a conventional automatic transmission having housings of an oil pump and an auxiliary gear means joined in contact with each other, the breathing system for the housing assembly is formed as including a breathing passage formed in the housing of the oil pump and a second breathing passage formed in the housing of the auxiliary gear means, wherein one end of said second breathing passage is connected with the first-mentioned breathing passage while the other end of said second passage is opened to the atmosphere. In this case, the breathing system composed of the two breathing passages is constructed so that the breathing passage extends from its inner end provided at a wall of the oil pump housing and opening toward the inner space to its outer end provided at a wall of the housing of the auxiliary gear means and opening toward the atmosphere in a manner of increasing its gravity potential to a relatively large extent when compared with the length of the breathing passage. Because of this arrangement in the conventional breathing system, even if oil enters into the breathing passge from said inner end, it is returned to the inner space from a middle portion of the passage due to the effect of gravity. Therefore, in conventional automatic transmissions, a problem such as oil leaking out from the outer end of the breathing passage has not been encountered.
However, when an automatic transmission incorporates an overdrive means therein which, in view of the required strength of gears, the required capacity of friction engaging elements, etc., is positioned between a fluid torque converter and an auxiliary gear means so as to accelerate rotating power which is not yet effected a torque increase by the auxiliary gear means, i.e., when an automatic transmission comprises a fluid torque conveter, an oil pump, an overdrive means and an auxiliary gear means assembled in series in said order, the housings of the oil pump and the auxiliary gear means are positioned apart from each other with interposition of the housing of the overdrive means therebetween, thereby making it difficult to construct a breathing system of the aforementioned conventional kind which has a simple structure and yet is free from any danger of causing oil leakage. In this case, it may be contemplated to provide a third breathing passage formed in the housing of the overdrive means so that the first breathing passage formed in the housing of the oil pump is connected with the second breathing passage formed in the housing of the auxiliary gear means by way of said third breathing passage. However, such a breathing system will include a relatively thin and long passage extending through the housing of the overdrive means, because the housing of the overdrive means is generally a relatively thin cylindrical housing which only allows a relatively small bore to be formed actually through the wall thickness thereof. Therefore, a breathing system which is laid out to include a passage portion which actually traverses the housing of the overdrive means necessarily becomes a long narrow passage system. On the other hand, an automatic transmission for automobiles is gnenerally positioned to rearwardly incline from its front end connected with the engine towards its rear end connected with a propeller shaft. Therefore, the breathing passage formed in the housing of the overdrive means so as to penetrate it, in its axial direction, is automatically inclined downward from its front end toward its rear end. If such a downwardly inclined narrow passage portion is included in the breathing passage system, oil which has entered into the inclined passage portion from its front end automatically flows toward the rear end portion where it stagnates. As a result, the breathing passage is filled with oil thereby losing its breathing function and finally causing leakage of oil therethrough.